Meanwhile, Optus was working to keep its mobile networks active in Toowoomba, Brisbane and the Lockyer Valley.

Three mobile towers at Helidon, Gatton and Gunters Hill are currently without power. Coverage is being supplemented to the area via additional towers.

Optus users in Brisbane is set to experience some congestion due to the sheer volume of voice and data traffic in the CBD, while Lockyer Valley towers are running on battery power after power was cut to the Mt Stradbroke facility.

"Due to the extreme flooding situation we are unable to access the area to install portable generators," Optus said, adding that a loss of service going into the evening was likely and would continue until the area became accessible to technicians.

"Some of our techs have been sandbagging and sleeping overnight at exchanges. St George telephone exchange was even wrapped in plastic to defend it from floodwaters," Telstra said.

St George exchange in Queensland, sandbagged and covered in plastic (Credit: Telstra)

Telstra said that evacuations in Brisbane were putting the network under strain, and advised users to only make calls if necessary and to keep them brief.

The telco said, however, that its biggest problem was access to mains power.

"The flooding has caused loss of power to Energex and we need them to power our base stations. Fortunately we've got battery backup so we're not experiencing issues. That could change in eight hours when those run down," Telstra said.

Telstra said it would replace depleted batteries and even install generators if required, but added that the clean-up and repair process could take weeks given that whole townships are still underwater.